Experiment 2 The Wound-Rotor Induction Motor Part II OBJECTIVE To determine the starting characteristics of the wound-rotor induction motor. To observe the rotor and stator currents at different motor speeds. DISCUSSION In the previous Experiment we saw that a considerable voltage appears across the rotor windings on open circuit, and that this voltage varies linearly with rotor slip in r/min, becoming zero at synchronous speed. If the rotor windings are short-circuited, the induced voltage will cause large circulating currents in the windings. To supply this rotor current, the stator current must increase in value above its ordinary exciting current level. The power consumed (VA) in the rotor windings (and associated circuitry) must be supplied by the stator windings. Therefore, we should expect the following: a) At standstill, or at low speed, the rotor currents, stator currents and torque will be high. b) At synchronous speed, the rotor current and torque will be zero, and the stator will only carry the exciting current. c) At any other motor speed, the currents and the developed torque will be between the above extremes. EQUIPMENT REQUIRED A Three-Phase Wound-Rotor Induction Motor, Electrodynamometer, Power Supply, AC Ammeter, and AC Voltmeter are required to perform this exercise. 2-1
PROCEDURE CAUTION! High voltages are present in this Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement! * 1. Using your Three-Phase Wound-Rotor Induction Motor, Electrodynamometer, Power Supply, AC Ammeter and AC Voltmeter, connect the circuit shown in Figure 2-1. Note that the three stator windings are connected to the variable 31 output of the power supply, terminals 4, 5 and 6. Figure 2-1. * 2. a. Couple the electrodynamometer to the motor with the timing belt. b. Connect the input terminals of the electrodynamometer to the fixed 120 V ac output of the power supply, terminals 1 and N. c. Set the dynamometer control knob at its full cw position (to provide a maximum starting load for the rotor). 2-2
* 3. a. Turn on the power supply and adjust for an E 1 of 100 V ac. The motor should be turning slowly. b. Measure and record the three rotor currents and the developed torque. I 1 = A ac, I 2 = A ac I 3 = A ac, Torque = N m c. Are the three rotor currents approximately equal? * Yes * No * 4. a. Gradually reduce the load on the motor by slowly adjusting the dynamometer control knob. As the load is reduced the motor speed will increase. b. Do the three rotor currents decrease as the motor speeds up? * Yes * No c. Do the three rotor currents decrease as the motor speeds up? * Yes * No d. Measure and record the rotor currents at a torque of 0.2 N m. I 1 = A ac, I 2 = A ac, I 3 = A ac e. Return the voltage to zero and turn off the power supply. * 5. a. Connect the circuit shown in Figure 2-2. Note that the fixed 31 output of the power supply, terminals 1, 2 and 3 are now being used. b. Set the dynamometer control knob at its full cw position (to provide a maximum starting load for the motor). 2-3
Figure 2-2. * 6. a. Turn on the power supply and quickly measure E 1, I 1, I 2 and the developed starting torque. Turn off the power supply. I 1 = A ac I 2 = A ac E 1 = V ac, Torque = N m b. Calculate the apparent power to the motor at starting torque. Apparent power = VA 2-4
REVIEW QUESTIONS 1. Assuming the full load 175 W motor speed is 1500 r/min, calculate the value of the full load torque using the formula for output power: P out (W) 2% x N x T 60 where P out = Mechanical Output Power in watts (W) N = Speed in revolution per minute (r/min) T = Torque in Newton meter (N m) = N m 2. Calculate the ratio of starting torque to full load torque: Torque ratio = 3. Assuming that the full load stator current is 1.2 A per phase, calculate the ratio of starting current to full load operating current. Current ratio = 4. If the stator voltage of a wound-rotor motor is reduced by approximately 50% of the rated value: a) By how much is the starting current reduced? = % 2-5
b) By how much is the apparent power reduced? = % c) By how much is the starting torque reduced? = % 2-6